Multicolor printers generate images which are composed of a plurality of different single-color images. The quality of the final multicolor image depends on the accuracy of the alignment of the individual images (also called “registration accuracy”). With the increasing resolution of modern printers the registration accuracy has become an issue of interest.
Different printing techniques are known. For example, in ink-jet printing droplets of liquid ink are directed from print heads towards a recording medium. Each print head has a plurality of ink channels. Pulses cause droplets of ink to be expelled as required from dot-forming elements in the form of orifices or nozzles at the end of the channels. These pulses are generated e.g. by thermal components in thermal ink-jet print heads or by piezo-electric elements in drop-on-demand print heads. Page-wide array ink-jet printers have an array of nozzles extending across the full width of the recording medium. The recording medium may be paper or any other suitable substrate to which the ink adheres, and is moved past the print heads by a conveyor formed, for example, by a belt or a drum.
The print heads are arranged in print stations which are typically transversely oriented to the conveyors advance direction and are spaced apart from each other in the advance direction. Due to the spaced arrangement of the print stations, the individual images are subsequently printed. If the distance between the print stations is smaller than the image length the individual images are printed in a staggered manner. Accordingly, the multicolor image to be printed is virtually separated into individual images to be printed by the respective print stations. In order to achieve registration of the images with respect to the advance direction (or longitudinal direction), the printing activity of the individual print stations is delayed until the image printed by the first print station arrives at the respective subsequent print station.
Assuming that the conveyor only moves the recording medium in the longitudinal direction, registration can be achieved by choosing the correct delays. However, small movements in a direction perpendicular to that may cause a lateral displacement of the recording medium from one print station to the other and, accordingly, a lateral misalignment of the individual images. Such lateral displacements may, for example, occur when the conveyor belt runs askew or performs oscillatory lateral movements.
In order to also achieve registration with respect to such lateral displacements, it is known to shift the image data to be printed by the individual print stations to compensate for this lateral displacement (see, for example, U.S. Pat. Nos. 5,587,771 and 6,335,748).
A printing device with a conveyor in the form of a rotating drum is known from U.S. Pat. No. 6,089,693. The printing device has a single print station. Due to large numbers of dot-forming elements (nozzles) in the print station, generally one or more of the nozzles will be defective. During a first pass (i.e. a first revolution of the drum), the print station prints the complete image, except for one or more columns corresponding to the defective nozzle or nozzles. Then the print station is laterally shifted, so that an operative nozzle is aligned to the original position of the defective nozzle. During a second pass (i.e. a second revolution of the drum) the missing column(s) is (are) printed.